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In this section you will find detailed information, including national overviews and state and regional level assessments, from the 2000-2002 National Land and Water Resources Audit theme assessments.
Australian Agriculture Assessment 2001
Australian agriculture assessment 2001
National Land and Water Resources Audit, 2001
APPENDIX 2. Australian Soil Resources Information System
Bulk density (topsoil and subsoil)
Bulk density is the weight of a dry soil in a unit of volume, and gives a measure of soil porosity.
Bulk density can indicate how the porosity (number of pore spaces) of soil samples helping to determine how much air or water can be stored and moved through the soil. Bulk density also indicates how tightly soil particles are packed and whether how difficult or easy the soil will be for roots (or shovels) to penetrate.
Soils with low bulk density are generally more suitable for agriculture, since the high pore space has a greater potential to store water and roots are able to grow more readily. As bulk density increases, resistance to roots increases and the amount of water available to crops decreases. Permeability of the soil also decreases so that crops are more susceptible to waterlogging. In sandy soils, bulk densities above 1.6 - 1.8 g/cm3 may cause problems with root penetration. In silty and clay soils, problems may arise at bulk densities above 1.4 g/cm3.
How does it vary and what is it related to?
The density of a soil sample is determined by the number of spaces (pores) in the sample, how tightly they are packed, and the composition of the solid material.
- Sandy soils have higher bulk densities (1.3 and 1.7 g/cm3) than finer-grained soils, because they have larger, but fewer, pore spaces.
- Fine-grained soils (silts and clays) have bulk densities between 1.1 and 1.6 g/cm3. In clay soils with good soil structure, there is a greater amount of pore space because the particles are very small, and many small pore spaces fit between them.
- Soils rich in organic matter generally have low bulk density.
- Surface soils usually have bulk densities in the range 1.1 - 1.4 g/cm3, decreasing to 0.9 - 1.2 g/cm3 after cultivation.
- Bulk density increases with compaction at depth, and very compact subsoils may have bulk densities above 2 g/cm3.
Table A12 Bulk density by land use categories for topsoil (g/cm3) by percent of land use type across Australia.
| < 1.2 (%) | 1.2 - 1.4 (%) | > 1.4 (%) | Total land use class area (ha) | |
|---|---|---|---|---|
| Conservation and natural environments | 7 | 76 | 17 | 263 824 100 |
| Production from native environments | 15 | 63 | 23 | 442 984 600 |
| Cropping | 12 | 60 | 28 | 22 512 900 |
| Grazing modified pasture | 11 | 51 | 38 | 19 223 000 |
| Horticulture | 19 | 47 | 34 | 350 600 |
| Irrigated cropping | 21 | 47 | 32 | 949 000 |
| Irrigated modified pasture | 8 | 62 | 30 | 1 079 000 |
| Total area | 750 923 200 |
Table A13 Bulk density by land use categories for subsoil (g/cm3) by percent of land use type across Australia*.
| < 1.2 | 1.2 - 1.4 | > 1.4 | Total land use class area (ha) | |
|---|---|---|---|---|
| Conservation and natural environments | 3 | 10 | 88 | 227 609 500 |
| Production from native environments | 3 | 24 | 73 | 426 254 500 |
| Cropping | 2 | 16 | 83 | 22 483 000 |
| Grazing modified pasture | 3 | 13 | 84 | 19 182 400 |
| Horticulture | 5 | 15 | 80 | 347 100 |
| Irrigated cropping | 2 | 26 | 72 | 947 400 |
| Irrigated modified pasture | 1 | 7 | 91 | 1 075 000 |
| Total area* | 697 898 900 |
* Different area compared with topsoil results from large areas having no estimate for subsoil bulk density.
How and why does it vary across Australia?
Areas with high organic matter (e.g. forests) generally have a low bulk density (< 1.1 g/cm3) in topsoils that increases to 1.2-1.3 g/cm3 in subsoils. These areas roughly correspond to Dermosols and Podosols. Vertosols have bulk densities less than 1.3 g/cm3 in topsoil and subsoil. Calcarosols and Chromosols have bulk densities around 1.4 g/cm3 throughout. Sodosols have bulk densities greater than 1.4 g/cm3 in topsoils, increasing to > 1.6 g/cm3 in subsoils.
The Australian Soil Resources Information System estimates for bulk density appear too low in the subsoil of Calcarosols and Sodosols across the wheatbelt of New South Wales and Victoria; actual bulk densities should be 1.6-1.7 g/cm3.
Level of uncertainty
The scale of the various soil maps used in deriving this map is shown in Figure A2.
The quality of estimates of bulk density for different soil groups varies widely. Bulk density data have not been collected in routine soil surveys despite their importance. The CSIRO database used by McKenzie et al. (2000) to draw interpretations of soil properties contained had bulk density determinations for only 1755 soil layers, and these were biased to soils used for agriculture and the Bago-Maragle forest soil survey study. Bulk density data are not available for many groups of soils and there are many instances where bulk density will have little if any correlation with generalised soil types (e.g. where land management practices have led to increases in bulk density across a range of soil types). Uncertainty associated with bulk density estimates is very high.
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